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71

Information Notice No. 93­71: Fire At Chernobyl Unit 2

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555

September 13, 1993

NRC INFORMATION NOTICE 93‐71: FIRE AT CHERNOBYL UNIT 2

Addressees

All holders of operating licenses or construction permits for nuclear power

reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information

notice to alert addressees to the fact that extensive fires may produce

unanticipated challenges such as happened at Unit 2 of the Chernobyl Nuclear

Power Station. It is expected that recipients will review the information for

applicability to their facilities and consider actions, as appropriate, to

avoid similar problems. However, suggestions contained in this information

notice are not NRC requirements; therefore, no specific action or written

response is required.

Description of Circumstances

On October 11, 1991, Unit 2 of the Chernobyl Nuclear Power Station was

operating at 70‐percent power. Each Chernobyl unit has two twin, independent

turbogenerators and all four units share a common turbine hall (see

Attachment 1). In order to perform minor repairs and adjustments on

turbogenerator 4 (TG‐4), operators reduced Unit 2 reactor power and isolated

the steam supply to TG‐4. At 7:46 p.m., the operators switched off the

generator excitation field and opened disconnect breakers 1, 2, and 3 in the

330 kV switchyard to electrically isolate TG‐4.

At 8:10 p.m., TG‐4 had coasted down to about 150 rpm when disconnect breaker 2

spuriously closed and reconnected TG‐4 to the electrical grid. The generator

rotor motorized on the reverse‐power condition and accelerated to synchronous

speed (3000 rpm) in under 30 seconds. The influx of current to TG‐4

overheated the conductor elements and caused a rapid degradation of the

mechanical end joints of the rotor and excitation windings. A centrifugal

imbalance developed and damaged generator bearings 10 through 14 and the seal

oil system allowing hydrogen gas and seal oil to leak from the generator

enclosure. Electrical arcing and frictional heat ignited the leaking hydrogen

and seal oil creating hydrogen flames 8 meters [27 feet] high and dense smoke

which obstructed the visibility of plant personnel.

When the burning oil reached the busbar of the generator it caused a
three‐phase 120,000 amp short circuit. Within 0.07 seconds, the generator

fault protective circuits sensed the short and opened disconnect breaker 2,

9309080104.

IN 93‐71

September 13, 1993

Page 2 of 3

which spuriously closed again 0.25 seconds later. The protection system

reopened the breaker 0.2 seconds after the closure and again the breaker

spuriously closed. Finally, protection circuits in the 330 kV electrical grid

disconnected the generator 0.27 seconds after the closure by opening a remote

breaker located in the town of Korosten, about 120 kilometers [72 miles] away.

Unit 2 control room operators shut down the reactor and initiated the maximum

allowable cooldown rate to achieve a safe reactor configuration.

Fire fighting efforts focused on containing the fire and preventing it from

spreading to adjacent equipment. There was little concern for the turbine

hall roof catching fire because the asphalt coating on the roof had been

removed after the 1986 accident at Chernobyl Unit 4. However, the local

ventilation systems, which were the only method available for heat and smoke

removal, were unable to cope with the smoke and heat generated by the fire.

The fire brigade was concerned for the structural integrity of the roof

because the roof supports had no heat retardant coating and because sprinkler

systems in the turbine hall were not designed to cool the structural supports.

The fire brigade attempted to cool the supports by spraying them with water

from below but were unsuccessful because the plant fire pumps could not

provide adequate flow to the area sprinkler systems and the local

fire‐fighting efforts at the same time. The metal trusses of the roof

structure reached a temperature greater than 900�C [1650�F] and collapsed at

8:35 p.m.

A 50 meter by 50 meter [165 feet by 165 feet] section of the roof collapsed

onto the turbine deck and also onto an adjacent pit that contained the main

feedwater pumps, the auxiliary feedwater pumps and their associated control

cabinets. Damage to the pump systems and a fire in one control cabinet

disrupted makeup water flow to the reactor cooling system. The operators cut

the electrical power to the pump motors and the control cabinets to remove

them as a potential ignition source. Because of the significant amount of

damage, the operators believed that the main feedwater and auxiliary feedwater

pumps could not be readily restored. Therefore, the operators added water to

the reactor primary coolant circuit by opening the steam relief valves to

reduce pressure and aligning a low‐pressure nonsafety‐grade condensate pump to

the auxiliary feedwater system piping. This arrangement made controlling the

steam separator drum water levels difficult but allowed the operators to

provide core cooling throughout the event. The fire was extinguished by

11:30 p.m.

Discussion

An investigation of the event later determined that a short circuit in the

control wiring for generator disconnect breaker 2 caused the repeated failures

of the breaker to remain open. This condition was critical because there was

no redundant on‐site isolation breaker for the generator output such as is

included in the more recent design of Chernobyl Unit 3.

.

IN 93‐71

September 13, 1993

Page 3 of 3
A notable safety significant aspect of the Chernobyl fire was the complete

loss of the main feedwater and auxiliary feedwater systems. Because redundant

safety components were not truly independent (i.e., there were no separate and

protected cubicles for the auxiliary feedwater pumps), a single structural

component failure led to the common failure of all feedwater pumps. The

proposed corrective actions include: (1) installation of three additional

emergency feed pumps outside of the turbine hall, (2) installation of an

automatic sprinkler system for the metal structure of the turbine hall and

roof, and (3) a review of the design of the smoke removal system in the

turbine hall and installation of a more effective system.

This event illustrates that extensive fires may place unanticipated loads on

fire protection systems and that structural failures could cause failures of

multiple trains of safety‐related systems and challenge the true separation,

independence, and redundancy of safety‐related components.

This information notice requires no specific action or written response. If

you have any questions about the information in this notice, please contact

the technical contact listed below or the appropriate Office of Nuclear

Reactor Regulation (NRR) project manager.

/S/'D BY BKGRIMES

Brian K. Grimes, Director

Division of Operating Reactor Support

Office of Nuclear Reactor Regulation

Technical contact: Eric J. Benner, NRR

(301) 504‐1171

Attachments:

1. Chernobyl Turbine Hall Layout

2. List of Recently Issued NRC Information Notices.

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